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A pinhole camera for monochromatic x-ray imaging
Volume 62, number 1
OPTICS COMMUNICATIONS
A PINHOLE CAMERA FOR MONOCHROMATIC
1 April 1987
X-RAY I M A G I N G
A.J. COLE, M.H. KEY, A. R I D G E L E Y Science and Engineering Research Council, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UK
D.A. BROWN, P.A. NORREYS, E.R. W O O D I N G Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 OEX, UK
and T.W. BARBEE l Stanford University, Stanford, CA 94305, USA
Received 22 September 1986; revised manuscript received 23 December 1986
An instrumental technique for recording monochromatic X-ray imagesof laser*produced plasmas has been developed using a Layered Synthetic Microstructure in combination with a pinhole camera. Wellresolvedimagesin 6 A X-ray emissionfrom 10 #m gold microdots are demonstrated.
The technique of monochromatic imaging of Xrays with two dimensional spatial resolution is one of considerable potential in the diagnostics of laser produced plasmas. A number of schemes have previously been investigated including diffraction from a single crystal double-reflection [ 1 ], a spherically curved crystal reflecting at normal incidence [2], Bragg reflection from a plane crystal combined with a pinhole camera [ 3 ] and an X-ray transmission gating incorporated into a reflection X-ray microscope [ 4 ]. XUV radiation has been imaged with a normal incidence spherical mirror coated with a layered synthetic microstructure [ 5 ]. We present here results obtained with a new method using a pinhole camera with a layered synthetic microstructure (LSM) or "Barbee" mirror [ 6 ] to provide a quasi-monochromatic X-ray image. The advantages of the LSM over crystals [ 3 ] are higher integral reflection coefficient, lower dispersion and lower resolution giving a brighter image with less intensity modulation. Present address: LawrenceLivermoreNational Laboratory,PO Box 808, Livermore,California 94550, USA. 0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
X-RAY PINHOLE CAMERA FOR MONOCHROMATIC IMAGING 5o/~o CHANNEL PLATE IMAGE INTENSIFIER
MONOCHROMATIC PLASMA PINHOLE ARRAY
IMAGE
z~" ~ ~ f /,,~ - ~, '~S~10pm PINHOLES
POLYCHROMATIC IMAGE 15cm
\\
CAMERA
PHOSPHOR/ FIBRE OPTIC
FILM
~_
WINDOW
Fig. 1. Intensified X-ray pinhole camera for monochromatic imaging. The layered synthetic microstructure consists of 60 layer pairs of carbon and palladium with an effective 2d spacing of 140 A. Its surface area is 9 x 60 mm. In the camera illustrated in fig. 1, pinholes are arranged to produce one polychromatic image (without reflection) and three quasi-monochromatic images by reflection from the LSM. Three pinholes are used to facilitate alignment. A tilt adjustment on the LSM controls the image position on the phosphor of the
Volume 62, number 1
OPTICS COMMUNICATIONS
1 April 1987
image intensifier detector system. The angle of incidence on the LSM is adjustable from 2 ° to 4 ° to reflect wavelengths of 5 to 10 ~k. The intensity of radiation reaching the front of the image intensifier in this configuration is E ( 2 ) d a 2 ( 1 + lira) 2 Eo ~
2xP
$2
R~
J cm 2 s-
TARGET /
/
t
~ J J
/
for plasma of diameter "s" emitting E(2) J cm 2 sper unit wavelength interval where a is the pinhole diameter, l is the distance from the plasma to the pinhole, m is the magnification of the camera, and
VYLAR BASE GOLD MICRODOT
Fig. 3. Microdot target (schematic).
SHADOW OF
BRAGG DIFFRACTED
BARBEE MIRROR
X-RAY SPECTRUM ¥
OF
DIFFRACTION AU
M
BY
X-RAYS CtPd
A
Vv',~ £3
MULTILAYER
"B~RBEE' WITH
1,5t
BAND
MIRROR d = 70 /~,
LASER
< 05
6~
~
~
ANSTR~M
2 5 2
=BRAGG ~NG~E D=~=ES:
5'7 2 33
Fig, 2. Bragg diffracted X-ray spectrum from "Barbee" mirror.
Volume 62, number 1
OPTICS COMMUNICATIONS
1 April 1987 0
J
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Volume 62, number 1
OPTICS COMMUNICATIONS
Rc is the integrated reflectivity of the mirror. The large value of d for the LSM relative to typical crystals gives greater image intensity. The theoretical integrated reflectivity Rc for unpolarised radiation and for first order diffraction is [ 7 ] Rc = (A/N) tan 0 tanh A
rad,
where 0 is the Bragg angle, where A is given by A = (Nd2/rt)(1 + Icos 201 )(OA --0B)N is the number of layer pairs and 0A and q~a are the scattering amplitude densities for X-rays in the layers of carbon and palladium respectiely. For the present LSM, Rc is calculated from the analytic formulae using ~A and OBvalues from [ 8 ] to be 1.7× 10 -2 rad for a wavelength of 6 A, which is ten times greater than for a high reflectivity plane crystal such as T1AP [ 7 ]. The angular full width half m a x i m u m for any line in the dispersed spectrum is [6] ZJ01/2 :
0.8882/Nd cos 0,
from this expression the spectral resolution of the C/Pd mirror was estimated to be ~2/2~ 1/34. The low spectral resolution smooths out fine structure in the quasi-continuum used to produce monochromatic images, giving better uniformity of response along the spectral dispersion axis of the image than with a crystal. The pinhole diameter controls the spatial resolution zlx which, in the geometrical limit is a(1 + 1/m). the smallest useful value of a occurs when diffraction begins to limit resolution, in this case when a = 6/zm and Ax= 6.2 #m. In the present experiment a = 10 + 1 p m and m = 3 5 . A 25/tm thick Be filter was used to suppress stray soft X-rays. An experiment to demonstrate the potential spatial resolution of the technique was conducted with laser pulses of 10 J, 1 ns duration and 0.53/tm wavelength which were focussed into a 100 #m diameter focal spot. Gold was chosen as suitable target emitting a broad M-band quasi-continuum in the range 5 to 60 [9]. The mirror angle was adjusted by replacing the pinhole with a horizontal edge to block the direct X-rays. X-rays then falling on the mirror were
4
1 April 1987
dispersed to form the M band emission spectrum in the centre of the detector as illustrated in fig. 2. The spectral resolution of the LSM is evidenced by the resolution of structure within the M band emission having 32/2~ 1/40 consistent with the calculated value. With the multiple pinholes in place, images were recorded from targets consisting of a line of 0.15/zm thick gold microdots (of dimensions 10 or 20 # m ) on a 125/zm thick Mylar substrate as illustrated in fig. 3. In the polychromatic image shown in fig. 4, the images from adjacent gold dots are resolved but the background radiation from the plastic substrate is significant. The simultaneously recorded image from the LSM shown in fig. 5 is of comparable brightness but the contrast of the gold plasma emission relative to that from the polymer substate is greatly enhanced. For instance, if the intensity of the background radiation to the right of the microdots in fig. 4 and 5 is compared to the peak intensity from the microdot itself, then the peak to background intensity ratio is 4 times greater in the case of the monochromatic image. The present results show the feasibility of using this method to study non uniformity of ablation on a spatial scale down to 20/zm, which is of considerable interest for research into energy transport in laser produced plasmas.
References
[ 1] B.S. Fraenkel, Appl. Phys. Lett. 41 (1982) 234. [2] L.M. Belyaev,A.B. Gil'vorg, Yu.A. Mikhailov, S.A. Pikuz, G.V. Sklizkov,A.Ya. Paenov and S.I. Fedolov,Sov.J. Quam. Electron. 6 (1976) 1121. [ 3] H. Azechi, S. Oda, T. Sasaki, T. Yamanaka and C. Yamanaka, Appl. Phys. Lett. 37 (1980) 998. [4] N.M. Ceglio,Nucl. Instrum. Methods 222 (1984) 111. [ 5 ] R. Benattar and J. Godart, Optics Comm. 5 ! (1984) 260. [ 6 ] T.W. BarbeeJr., AIP Conf. Proc. 75 (1981 ) 13!. [7] J.H. Underwood and T.W. Barbee Jr., AIP Conf. Proc. 75 (1981) 170. [ 8] B.L.Henke,P. Lee,T.J. Tanaaka,R.L. Shimabukuroand B.K. Fujikawa, AIP Conf. Proc. 75 (1981) 340. [9] M. Buiquet, D. Pain, J. Bauche and E. Luc-Koenig, Physica Scripta 31 (1985) 137.
OPTICS COMMUNICATIONS
A PINHOLE CAMERA FOR MONOCHROMATIC
1 April 1987
X-RAY I M A G I N G
A.J. COLE, M.H. KEY, A. R I D G E L E Y Science and Engineering Research Council, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, UK
D.A. BROWN, P.A. NORREYS, E.R. W O O D I N G Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 OEX, UK
and T.W. BARBEE l Stanford University, Stanford, CA 94305, USA
Received 22 September 1986; revised manuscript received 23 December 1986
An instrumental technique for recording monochromatic X-ray imagesof laser*produced plasmas has been developed using a Layered Synthetic Microstructure in combination with a pinhole camera. Wellresolvedimagesin 6 A X-ray emissionfrom 10 #m gold microdots are demonstrated.
The technique of monochromatic imaging of Xrays with two dimensional spatial resolution is one of considerable potential in the diagnostics of laser produced plasmas. A number of schemes have previously been investigated including diffraction from a single crystal double-reflection [ 1 ], a spherically curved crystal reflecting at normal incidence [2], Bragg reflection from a plane crystal combined with a pinhole camera [ 3 ] and an X-ray transmission gating incorporated into a reflection X-ray microscope [ 4 ]. XUV radiation has been imaged with a normal incidence spherical mirror coated with a layered synthetic microstructure [ 5 ]. We present here results obtained with a new method using a pinhole camera with a layered synthetic microstructure (LSM) or "Barbee" mirror [ 6 ] to provide a quasi-monochromatic X-ray image. The advantages of the LSM over crystals [ 3 ] are higher integral reflection coefficient, lower dispersion and lower resolution giving a brighter image with less intensity modulation. Present address: LawrenceLivermoreNational Laboratory,PO Box 808, Livermore,California 94550, USA. 0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
X-RAY PINHOLE CAMERA FOR MONOCHROMATIC IMAGING 5o/~o CHANNEL PLATE IMAGE INTENSIFIER
MONOCHROMATIC PLASMA PINHOLE ARRAY
IMAGE
z~" ~ ~ f /,,~ - ~, '~S~10pm PINHOLES
POLYCHROMATIC IMAGE 15cm
\\
CAMERA
PHOSPHOR/ FIBRE OPTIC
FILM
~_
WINDOW
Fig. 1. Intensified X-ray pinhole camera for monochromatic imaging. The layered synthetic microstructure consists of 60 layer pairs of carbon and palladium with an effective 2d spacing of 140 A. Its surface area is 9 x 60 mm. In the camera illustrated in fig. 1, pinholes are arranged to produce one polychromatic image (without reflection) and three quasi-monochromatic images by reflection from the LSM. Three pinholes are used to facilitate alignment. A tilt adjustment on the LSM controls the image position on the phosphor of the
Volume 62, number 1
OPTICS COMMUNICATIONS
1 April 1987
image intensifier detector system. The angle of incidence on the LSM is adjustable from 2 ° to 4 ° to reflect wavelengths of 5 to 10 ~k. The intensity of radiation reaching the front of the image intensifier in this configuration is E ( 2 ) d a 2 ( 1 + lira) 2 Eo ~
2xP
$2
R~
J cm 2 s-
TARGET /
/
t
~ J J
/
for plasma of diameter "s" emitting E(2) J cm 2 sper unit wavelength interval where a is the pinhole diameter, l is the distance from the plasma to the pinhole, m is the magnification of the camera, and
VYLAR BASE GOLD MICRODOT
Fig. 3. Microdot target (schematic).
SHADOW OF
BRAGG DIFFRACTED
BARBEE MIRROR
X-RAY SPECTRUM ¥
OF
DIFFRACTION AU
M
BY
X-RAYS CtPd
A
Vv',~ £3
MULTILAYER
"B~RBEE' WITH
1,5t
BAND
MIRROR d = 70 /~,
LASER
< 05
6~
~
~
ANSTR~M
2 5 2
=BRAGG ~NG~E D=~=ES:
5'7 2 33
Fig, 2. Bragg diffracted X-ray spectrum from "Barbee" mirror.
Volume 62, number 1
OPTICS COMMUNICATIONS
1 April 1987 0
J
£
o ~s
0
f °=
o
.o
0
e,~
0
909 3AOSV
AIlSN30
0
ii~i~:i~ii~ ~i~ %~ ~ ~i~ i~ ~:~ ~?~ /~ ~ ~!~ ~i? i~ ~!~!~ ~ ~?~ ~ / i i i ii ¸
g
~S
0
"4 e~ I
!~)!iiiiiiii~i~i~!~i~ !~ ~
!
~/ ~ / i i ¸¸i ~
0
OOJ
3AOSV AIISN30
Volume 62, number 1
OPTICS COMMUNICATIONS
Rc is the integrated reflectivity of the mirror. The large value of d for the LSM relative to typical crystals gives greater image intensity. The theoretical integrated reflectivity Rc for unpolarised radiation and for first order diffraction is [ 7 ] Rc = (A/N) tan 0 tanh A
rad,
where 0 is the Bragg angle, where A is given by A = (Nd2/rt)(1 + Icos 201 )(OA --0B)N is the number of layer pairs and 0A and q~a are the scattering amplitude densities for X-rays in the layers of carbon and palladium respectiely. For the present LSM, Rc is calculated from the analytic formulae using ~A and OBvalues from [ 8 ] to be 1.7× 10 -2 rad for a wavelength of 6 A, which is ten times greater than for a high reflectivity plane crystal such as T1AP [ 7 ]. The angular full width half m a x i m u m for any line in the dispersed spectrum is [6] ZJ01/2 :
0.8882/Nd cos 0,
from this expression the spectral resolution of the C/Pd mirror was estimated to be ~2/2~ 1/34. The low spectral resolution smooths out fine structure in the quasi-continuum used to produce monochromatic images, giving better uniformity of response along the spectral dispersion axis of the image than with a crystal. The pinhole diameter controls the spatial resolution zlx which, in the geometrical limit is a(1 + 1/m). the smallest useful value of a occurs when diffraction begins to limit resolution, in this case when a = 6/zm and Ax= 6.2 #m. In the present experiment a = 10 + 1 p m and m = 3 5 . A 25/tm thick Be filter was used to suppress stray soft X-rays. An experiment to demonstrate the potential spatial resolution of the technique was conducted with laser pulses of 10 J, 1 ns duration and 0.53/tm wavelength which were focussed into a 100 #m diameter focal spot. Gold was chosen as suitable target emitting a broad M-band quasi-continuum in the range 5 to 60 [9]. The mirror angle was adjusted by replacing the pinhole with a horizontal edge to block the direct X-rays. X-rays then falling on the mirror were
4
1 April 1987
dispersed to form the M band emission spectrum in the centre of the detector as illustrated in fig. 2. The spectral resolution of the LSM is evidenced by the resolution of structure within the M band emission having 32/2~ 1/40 consistent with the calculated value. With the multiple pinholes in place, images were recorded from targets consisting of a line of 0.15/zm thick gold microdots (of dimensions 10 or 20 # m ) on a 125/zm thick Mylar substrate as illustrated in fig. 3. In the polychromatic image shown in fig. 4, the images from adjacent gold dots are resolved but the background radiation from the plastic substrate is significant. The simultaneously recorded image from the LSM shown in fig. 5 is of comparable brightness but the contrast of the gold plasma emission relative to that from the polymer substate is greatly enhanced. For instance, if the intensity of the background radiation to the right of the microdots in fig. 4 and 5 is compared to the peak intensity from the microdot itself, then the peak to background intensity ratio is 4 times greater in the case of the monochromatic image. The present results show the feasibility of using this method to study non uniformity of ablation on a spatial scale down to 20/zm, which is of considerable interest for research into energy transport in laser produced plasmas.
References
[ 1] B.S. Fraenkel, Appl. Phys. Lett. 41 (1982) 234. [2] L.M. Belyaev,A.B. Gil'vorg, Yu.A. Mikhailov, S.A. Pikuz, G.V. Sklizkov,A.Ya. Paenov and S.I. Fedolov,Sov.J. Quam. Electron. 6 (1976) 1121. [ 3] H. Azechi, S. Oda, T. Sasaki, T. Yamanaka and C. Yamanaka, Appl. Phys. Lett. 37 (1980) 998. [4] N.M. Ceglio,Nucl. Instrum. Methods 222 (1984) 111. [ 5 ] R. Benattar and J. Godart, Optics Comm. 5 ! (1984) 260. [ 6 ] T.W. BarbeeJr., AIP Conf. Proc. 75 (1981 ) 13!. [7] J.H. Underwood and T.W. Barbee Jr., AIP Conf. Proc. 75 (1981) 170. [ 8] B.L.Henke,P. Lee,T.J. Tanaaka,R.L. Shimabukuroand B.K. Fujikawa, AIP Conf. Proc. 75 (1981) 340. [9] M. Buiquet, D. Pain, J. Bauche and E. Luc-Koenig, Physica Scripta 31 (1985) 137.